Electrocatalytic CO₂ Reduction to Formate with Molecular Fe(III) Complexes Containing Pendent Proton Relays

Previously, we reported an iron­(III) complex with 6,6′-([2,2′-bipyridine]-6,6′-diyl)­bis­(2,4-ditertbutyl-phenol) as a ligand (Fe­(^tbudhbpy)­Cl, 1) as catalytically competent for the electrochemical reduction of CO₂ to formate (Faradaic efficiency FE_HCO2^– = 68 ± 4%). In mechanistic experiments, an essential component was found to be a pre-equilibrium reaction involving the association of the proton donor with the catalyst, which preceded proton transfer to the Fe-bound O atoms upon reduction of the Fe center. Here, we report the synthesis, structural characterization, and reactivity of two iron­(III) compounds with 6,6′-([2,2′-bipyridine]-6,6′-diyl)­bis­(2-methoxy-4-methylphenol) (^mecrebpy­[H]₂, Fe­(^mecrebpy)­Cl, 2) and 6,6′-([2,2′-bipyridine]-6,6′-diyl)­bis­(4-(tert-butyl)­benzene-1,2-diol) (^tbucatbpy­[H]₄, Fe­(^tbucatbpy), 3) as ligands, where pendent −OMe and −OH groups are poised to modify the protonation reaction involving the Fe-bound O atoms. Differences in selectivity and activity for the electrocatalytic reduction of carbon dioxide (CO₂) to formate (HCO₂^–) between complexes 1–3 were assessed via cyclic voltammetry and controlled potential electrolysis (CPE) experiments in N,N-dimethylformamide. Mechanistic studies suggest that the O atoms in the secondary coordination sphere are important for relaying the exogenous proton donor to the active site through a preconcentration effect, which leads to the J_HCO2⁻ (partial catalytic current density for formate) increasing by 3.3-fold for 2 and 1.2-fold for 3 in comparison to the J_HCO2⁻ of ₁. These results also suggest that there is a difference in the strength of the interaction between the pendent functional groups and the sacrificial proton donor between 2 and 3, resulting in quantifiable differences in catalytic activity and efficiency. CPE experiments demonstrate an increased FE_HCO2^– = 85 ± 2% for 2, whereas 3 had a lower FE_HCO2^– = 71 ± 3%, with CO and H₂ generated as co-products in each case to reach mass balance. These results indicate that using secondary sphere moieties to modulate metal–ligand interactions and multisite electron and proton transfer reactivity in the primary coordination sphere through reactant preconcentration can be a powerful strategy for enhancing electrocatalytic activity and selectivity.